Hello Charlie...

You are right about Juan de la Cierva and his perfection of the autogyro in the early 20's. In the Townsend book, Cierva's early efforts are pretty well documented. His rigidly mounted rotors (no flap, no teeter, etc.) all resulted in flipping over as the aircraft gained forward speed. Later he went to models and almost by accident one of his models used feathers as the rotor. Much to his surprise (and probably satisfaction) the plane flew just fine. He then reasoned that the feathers were able to flex up and down and that neutralized the uneven lift between the forward-going and rearward-going blades.

Moving to full size gyroplanes, it may be (and I emphasize 'may') that the blades being much longer than our model blades and as a result, more flexible, actually bend up (or flap) to achieve the same effect that Cierva saw with his feathered models---even though the gyroplane rotors are teetering. ( By the way, the "Whistler" (the designer's name escapes me)
published in RCM in 1991 uses a teetering rotor, but it also has stub wings with upturned tips. It flys fast. I built one and had something like 65 takeoffs before I had a successful landing.
I attribute that more to my flying skill than the aircraft design!)

Well, I hope this adds to the continuation of this interesting discussion!

Bill

P.S. Maybe we could arrange an after dinner 'technical discussion' at the Muncie Fly-in.